Many countries of the world are becoming cognizant of the detrimental effect on the environment of uncontrolled discharge of heavy metals and cyanides of metals. Cyanides have been known as potent poisons for many years.
A simple continuous process for destruction of cyanides and heavy metal precipitation would allow many small plating operators to treat their waste streams on-site and recover the heavy metals for recycle or for safe landfill. All common heavy metals except hexavalent chromium easily precipitate as hydroxides with optimum alkalinity for individual precipitation varying from a pH of about 7.5 to 13.
This invention encompasses the use of excess ferrous sulfate to reduce hexavalent chromium to trivalent chromium which then may be precipitated along with cadmium, zinc, arsenic, ferric iron, lead, nickel, copper, etc., as hydroxides. Further, we have found that all of these metals will coprecipitate completely with ferric iron, as hydroxides, at a pH of 9.5, to leave very low levels of heavy metals in solution. Our process combines a hypochlorite oxidation to destroy cyanides in a manner to allow destruction of excess hypochlorite by ferrous sulfate oxidation, with more ferrous sulfate then being added for hexavalent chromium reduction. These reactions occur simultaneously in one reactor at pH of approximately 9.5. In this manner ferric ions to allow optimum hydroxide formation to co-precipitate with the other heavy metals are also formed and variations in amount of hexavalent chromium are accomodated.
We have considered the following patents:
______________________________________ Patent No. Inventor Date ______________________________________ 3,931,007 Sugano et al. 1/6/1976 DL. 85032 R Schulz, G. 8/12/1970 4,343,706 Etzel et al. 8/10/1982 4,169,053 Sakakibaro et al. 9/25/1979 82354w/50 Takeda Chem. Ind. 11/5/1973 ______________________________________
None of the above patents cover an integrated process for both cyanide destruction and heavy metal removal.